Field of the Invention
The present invention relates to a focusing method, focusing apparatus, exposure apparatus, and device manufacturing method.
Description of the Related Art
A microdevice such as a semiconductor device or liquid crystal display device is manufactured by a photolithography method of transferring, onto a photosensitive substrate, a pattern formed on a mask. An exposure apparatus used in the photolithography process includes a mask stage which supports a mask, and a substrate stage which supports a substrate. While sequentially moving the mask stage and substrate stage, the exposure apparatus projects and transfers, onto a substrate via a projection optical system, a pattern formed on a mask. In the exposure apparatus, every time each exposure target region (shot region) on a substrate is moved to an exposure position below the projection optical system by the substrate stage, a surface position measurement device detects a shift between the focus of the projection optical system and the surface position of the substrate to correct the surface position of the substrate and focus on it.
Japanese Patent Laid-Open No. 4-354320 discloses an oblique incidence surface position measurement device which detects a surface position by irradiating a surface to be detected with a beam from an oblique direction at an angle almost parallel to the surface. The oblique incidence surface position measurement device includes a light projecting unit which is arranged on one side with respect to a projection optical system and emits a beam, and a light receiving unit which is arranged on the other side and receives reflected light of the beam reflected by the surface to be detected. The purpose of irradiation with a beam at an angle almost parallel to the surface to be detected is to accurately focus the projection optical system on the substrate surface by using only light reflected by the surface of a photoresist (photosensitive agent) applied on the substrate. If the angle with respect to the surface to be detected is large, the irradiation light is refracted, enters the photoresist, reaches an underlying oxide film layer or a layer below it, and is reflected by the layer. As a result, a surface of the substrate that is being measured becomes unclear.
When the oblique incidence surface position measurement device is used, a high-accuracy detection result can be obtained without the influence of the projection optical system, compared to, for example, a TTL (Through The Lens) surface position measurement device via the projection optical system. In the projection optical system, aberration is satisfactorily corrected for only the wavelength of exposure light. Thus, when detection light is different in wavelength from exposure light in the TTL surface position measurement device, aberrations occur. In contrast, when surface position detection light is equal in wavelength to exposure light, the photoresist applied on the substrate is undesirably exposed to light. To avoid this, an exposure apparatus for manufacturing a microdevice often employs the oblique incidence surface position measurement device.
When manufacturing a microdevice such as a MEMS, a structure is sometimes formed on a substrate having an uneven shape. When exposing the bottom of the recessed portion of the substrate in order to form a structure in the recessed portion of the substrate, if the projection optical system is focused on the bottom of the recessed portion serving as the exposure target region, detection light may be reflected by the projecting portion in the oblique incidence surface position measurement device. In this case, detection light passing through the exposure target region cannot be obtained, and the oblique incidence surface position measurement device may not be able to focus on the exposure target region. Also, when the projecting portion exists at the periphery of the exposure target region, the oblique incidence surface position measurement device may not be able to focus on the exposure target region owing to the presence of the projecting portion.